1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device.
2. Description of the Related Art
Generally, a liquid crystal display device includes a liquid crystal module (hereinafter, “LCM”), a driving circuit for driving the LCM and a case. The LCM includes a liquid crystal display panel having liquid crystal cells arranged in a matrix between two substrates, a backlight unit for irradiating light to the liquid crystal display panel. The LCM also includes an optical sheet for directing light from the backlight unit into a vertical direction toward the liquid crystal display panel, a lower polarizing plate at a lower portion of the liquid crystal display panel for polarizing light emitted from the backlight unit in a first direction and an upper polarizing plate at an upper portion of the liquid crystal display panel for polarizing light emitted from the liquid crystal display panel in a second direction perpendicular to the first direction. Such a liquid crystal display panel, a backlight unit and an optical sheet work together to prevent a light loss. The case encloses and protects the outside of the LCM to prevent damage to the LCM by an outside force.
FIG. 1 is a cross-sectional view of the related art liquid crystal module. Referring to FIG. 1, a liquid crystal module 1 includes a liquid crystal display panel 2 having a liquid crystal cell matrix, an upper polarizing plate 42 is positioned at a front surface of the LCM and a lower polarizing plate 40 is positioned at a rear surface of the liquid crystal display panel 2. A backlight unit 3 is positioned under the lower portion of the lower polarizing plate 40.
The liquid crystal display panel 2 includes a thin film transistor array substrate 2a and a color filter array substrate 2b adhered to each other with a liquid crystal layer (not shown) between the two substrates. The thin film transistor array substrate 2a includes signal lines (not shown) and thin film transistors (not shown). The color filter array substrate 2b includes color filters (not shown) and a black matrix (not shown).
The backlight unit 3 includes a lamp 20 for generating light, a lamp housing 10 for enclosing the lamp 20, a light guide plate 24 for distributing light from the lamp 20 across the liquid crystal display panel 2, a reflector 26 at the rear surface of the light guide plate 24, and diff-using sheets 30 sequentially built on the light guide plate 24. The lower polarizing plate 40 is attached to the rear surface of the thin film transistor array substrate 2a so that the liquid crystal display panel 2 can receive polarized light from the backlight unit 3, and the upper polarizing plate 42 is attached to the front surface of the color filter array substrate 2b to emit polarized light from the liquid crystal display panel 2.
FIG. 2 is a diagram for explaining a structure of the related art polarizing plate and the viewing angle compensation principle by the compensation film of the polarizing plate. FIG. 2 is a diagram of the related art polarizing plates 40 and 42 coupled with a liquid crystal display panel. As shown in FIG. 2, the related art polarizing plates 40 and 42 have a structure in which the compensation film (d-LC: discotic-LIQUID CRYSTAL) 40b and 42b, a first protective layer (wide view tree acetate cellulose: Wv-TAC) 40c and 42c, a polarizer (Poly Vinyl Alcohol: PVA) 40d and 42d and a second protective layer 40e and 42e are sequentially built on each of the upper and lower substrates 2a and 2b of the liquid crystal display panel. The compensation films 40b and 42b are attached to the outer surfaces of the substrates 2a and 2b of the liquid crystal display panel by adhesive 40a and 42a. 
The compensation film 42b compensates for a viewing angle decline by orienting liquid crystal molecules 50 in the compensation film 42b to be symmetrical with respect to liquid crystal molecules 51 at an area A of an upper portion of the liquid crystal layer adjacent to an alignment film 53 in the liquid crystal display panel. As shown in FIG. 2, whenever an electric field is applied to liquid crystal molecules 51 (that is, a TN mode), a misalignment is generated in the liquid crystal molecules 51 at an area A of the liquid crystal display panel by an alignment force of the alignment film 53 inside of the upper substrate 2b. The decline of the viewing angle by such an improper liquid crystal alignment in the area A is prevented by the compensation film 42b including liquid crystal layer molecules 50 aligned in such a manner as to have symmetry with the liquid crystal molecules 51 in the misaligned liquid crystal area A. Compensation film 40b also including liquid crystal layer molecules aligned in such a manner as to have symmetry with the liquid crystal molecules that are improperly aligned adjacent to the alignment film 54 inside of the upper substrate 2a. 
First protective layers 40c and 42c and second protective layers 40e and 42e are respectively positioned at the upper/lower surfaces of the polarizers 40d and 42d to protect the polarizers 40d and 42d. The first protective layers 40c and 42c also assists in the functions of the compensation films 40b and 42b in the role of viewing angle compensation. The polarizers 40d and 42d are a Poly Vinyl Alcohol film coated with iodine and stretched in a specific direction. The light is absorbed in the stretched direction and penetrates in the direction that is perpendicular to the stretched direction so as to polarize the light.
FIG. 3 is a diagram representing a transformation of each layer by a shear stress generated between a static upper substrate and a contracting force of a polarizer. After the polarizer 42d is stretched in a designated direction, it is then affixed in the stretched state by the first and second protective layers 42c and 42e so as to have a light polarization capability. But such a polarizer 42d has a tendency to contract in a direction opposite to that of the stretched direction when subjected to either or both internal heat from the backlight unit and external heat. As a result, the first protective layer 42c and the compensation film 42b also contract in a direction opposite to the stretched direction due the contracting polarizer 42d, as shown in FIG. 3. Because a lower area of the compensation film 42b is firmly attached by an adhesive layer 142a to the upper substrate 2b while an upper area of the compensation film 142b is affected by a contracting force transmitted through the first protective layer 142c from the polarizer 142c, a strong shear stress is generated at the lower area of the compensation film 142b. Due to the shear stress, a refraction index anisotropy is generated in the liquid crystals within the compensation film 142b. 
FIG. 4 is a diagram representing a change in optical axial direction of the liquid crystal molecules within a compensation film by shear stress on the compensation film. FIG. 5 shows a related art liquid crystal module exhibiting a light leakage phenomenon due to a contracted polarization film. As shown in FIG. 4, the liquid crystal molecule has a steady state optical axial direction before a heat contraction. However, the optical axial direction of the liquid crystal molecule diverges from the steady state position due shear stress caused by a contraction at the upper portion of the compensation film 42b, so that an optical axial direction of the liquid crystal is changed. Thus, the initial alignment of the liquid crystal molecules 51 positioned within the compensation film 42b is disturbed and thus can no longer sufficiently compensate for the improperly aligned liquid crystal molecules of the liquid crystal display panel. As a result, light leakage is generated while realizing an image so as to cause a reduction in picture quality, as shown in FIG. 5.